In contrast to thermoplastic polymers, which can be transformed and re-transformed using heat which, with or without a contribution of shearing mechanical energy, softens them and allows them to flow, thermosetting polymers constitute chemically crosslinked polymer networks, that is to say crosslinked by irreversible crosslinking bonds of covalent type, which, once obtained, can no longer be transformed by the action of heat. A thermosetting resin, once the polymer network has been formed, becomes a thermoset polymer network which will not flow under the effect of heat, even with a contribution of shearing mechanical energy. It is habitually said that a thermoset polymer, subjected to the effect of a constant increase in temperature, will end up by decomposing before being able to flow, as a result of the robustness of the crosslinking network formed by covalent bonds. There exists numerous systems which can result in the production of thermoset crosslinked networks involving a great variety of possible chemistries, such as, for example epoxy, polyurethane, phenol/formaldehyde, melamine/formaldehyde, silicone, urea/formaldehyde, polyester or unsaturated polyester networks. The chapter entitled “Polymer Networks” by Karel Dusek and Miroslava Duskova-Smrckova, in volume 3 of the series Macromolecular Engineering [Wiley-VCH, 2007, eds. K. Matyjaszewski, Y. Gnanou and L. Leibler], cites the main crosslinkable systems which make it possible to obtain crosslinked polymer networks.
It is customary for a person skilled in the art to consider that a thermoset polymer network is obtained by the mixing and consequent reaction of at least two components, with at least one of the two having a functionality greater than 2 with regard to the reaction involved. It is also customary for a person skilled in the art, in particular in the case of systems of epoxy or polyurethane type, to call the component carrying epoxy or isocyanate functional groups “the resin” and to call the component carrying amine or alcohol functional groups “the curing agent”. Another type of language which may be encountered in this field is that which assigns the name “resin” also to the polymer network being formed or to the final polymer network. Thus, it is not uncommon to hear talk of, for example, epoxy, polyurethane or polyester resins. As regards epoxy systems in particular, it is therefore necessary to take into account the context in order to determine whether the name epoxy resin applies to the compound carrying the starting oxirane reactive functional groups (hereinafter denoted base epoxy resin) or whether it concerns the final network, after reaction with a curing agent.
The curing agent, according to this name, is thus a compound, often a polyfunctional compound, carrying reactive amine or alcohol units. It is possible to include, as a mixture with this curing agent, inter alia, compounds which are inert with regard to the reaction (such as solvents) or, on the contrary, reactive solvents or diluents which make it possible to control the reaction and to adjust certain mechanical properties of the final product, and also catalysts which make it possible to accelerate the crosslinking of the reactive components.